Managing a lot of tags is difficult. You can group tags but you cannot prevent the group to be set as a tag.
In most cases, it is okay or even wanted, but I have some groups of tags, where I dont want the group to be selected as tag.

e.g. you have a taglist like this

- Places
  - Cities
    - London
  + Streets
  + Landmarks
  ...

I dont want "Places", "Streets" and "Cities" to be selectable as a tag because I only want to tag where the picture was taken.
Also I dont want to see the grouping in tags being removed but added a "tag" that cannot be selected. If the default icon would be a folder-icon it would be perfekt.